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On 01/03/2011 at 12:43, xxxxxxxx wrote:

ok, but then one of them must be wrong, right ?
or at least less accurate than the other.

I did a little test by pushing an object along the two different splines with the align to spline tag:
When printing out that objects distance from the center, it seems that the circle primitive is closer to a perfect circle than the arc primitive, which has a greater variation from 100.0 than the circle primitive.

I'm using the arc primitive in my WindowGenerator plugin , so naturally im interested in getting as precise as possible, and trying to figure out where the error is.

The best you could do would be to increase the number of control points that define your circle/arc to reduce the margin of error, like what Lennart just mentioned.
SinCos functions to arrange the points perfectly, then hope that the bezier interpolation generated would be close enough for your tolerances.